The invention relates generally to permanent magnet electrical machines and, more particularly, to a system and method for implementing a post-fault protection scheme for preventing demagnetization of permanent magnets in such electrical machines.
The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. One such type of electrical machine that is commonly used in such industries is permanent magnet (PM) machines—which are electrical machines with permanent magnets positioned in/on the machine to provide the magnetic field against which the rotor field interacts to produce torque in the electrical machine. In most standard PM machines, i.e., PM synchronous machines, the permanent magnets are positioned on the rotor and are either surface mounted on or embedded inside a lamination stack of the rotor. In stator PM machines, the permanent magnets and windings are located in the stator instead of the conventional rotor permanent magnet topology, with examples of stator PM machines including permanent magnet flux switching machines, flux reversal machines, and doubly-salient permanent magnet machines.
One drawback to PM electrical machines is the prohibitive costs of the materials used to form the permanent magnets, as permanent magnets are often formed at least partially of expensive rare-earth elements, with neodymium magnets being a common example. To reduce costs, ceramic permanent magnets may be employed as an alternative to rare-earth permanent magnets. Ceramic magnets are cheap and can be used to improve the performances of PM synchronous machines or stator PM machines. However, ceramic permanent magnets exhibit little resistance to demagnetization against the high demagnetizing magnetic fields that arise in the event of an internal fault in the motor windings. Once the magnets are demagnetized, both the stator and the rotor of the machine must be scrapped. Moreover, a sudden fault in the motor might also lead to damages to the process the machine is associated with, because of the uncontrolled halt of such process.
Therefore, it would be desirable to provide a system and method for preventing the demagnetization of low cost permanent magnets (e.g., ceramic permanent magnets) after a fault. Prevention of such demagnetization allows for the electrical machine to be protected while the process it is connected to is brought to a halt in a safe and controlled manner, with the damage to the electrical machine thus being limited only to the windings thereof, which can relatively easily be reconstructed.